COPENHAGEN, Denmark — A DDoS cyber attack made buying a ticket tough for Danish rail travellers Sunday night. A Distributed Denial of Service (DDoS) attack is an attempt to make an online service unavailable by overwhelming it with traffic from multiple sources. The hack made it impossible to purchase a ticket via the DSB app, on the website, at ticket machines and in 7-Eleven kiosks at the stations. The internal mail and telephone systems used by DSB employees were also affected by the hack.